C4 Cruise-Control Repair - No-Cruise Blues

How To Diagnose And Repair C4 Cruise-Control Problems

Since the inception of our Tech Corner column last year, you've been keeping us busy with questions on the maintenance and repair of your modern and vintage Corvettes. We've found that many of your inquiries are similar in subject matter, with queries on cruise-control systems being near the top of the list. The following email is representative of those questions.

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I've read your magazine for several years, and your technical articles have bailed me out many times. I like to think of myself as a shade-tree mechanic, so when my 35,000-mile '92 Corvette came out of winter storage this year with a dead battery, I thought it was no big deal. Well, since I installed the battery, the cruise control has stopped working. Do you think I caused a voltage spike and blew one of the modules? Is there anything I can do, other than take the car to the dealer?
Peter

There's a common scenario that occurs when installing a new battery in a C4 Corvette. If you look at your cruise-control servo, which is located just above the battery, you'll see two vacuum lines coming off of it. The act of sliding the battery into the tray can break these lines or dislodge them from the servo. Remember: Even though your Corvette only has 35,000 miles on the odometer, it's still almost 20 years old. At that age, the vacuum hoses have probably become brittle and are prone to cracking. Examine them and replace as necessary. Assuming the hoses appear OK, it's time to dig a little deeper.

A Closer Look at Cruise-Control Systems
The cruise-control system in most GM cars operates a mechanical linkage to the throttle body by using a vacuum motor called the cruise servo. This servo has a diaphragm, which is moved when vacuum is applied to one side. There are two solenoids located inside the servo. One connects the vacuum motor to the vacuum tank and allows for the application of more throttle. The other solenoid vents the vacuum to reduce throttle position.

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On the '84-'89 Corvettes, these solenoids are controlled by the instrument panel (IP), which controls vehicle speed (throttle position) by pulsing the two solenoids on and off. Out of all the cruise-control problems I've repaired through the years, very few have had to do with a faulty IP. In the '90 and newer C4 models, the cruise-control module is a stand-alone unit mounted either on the right side of the instrument panel or below the passenger-side dash.

There are a few common problems to look for on the C4 cruise-control systems. Let's go through how to test a system with minimal tools. All you'll need is a test light, a multimeter, a vacuum pump with gauge, and a little knowledge.

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C4 Cruise-Control Repair - No-Cruise Blues

If your cruise control won't work at all, the next step is to make sure the servo operates freely. While squeezing the diaphragm with your hand, watch the throttle open and close. The cable should move freely, with no binding or tight spots.

The first step when working with equipment that may be controlled by a fuse is to check all of the fuses with a test light. First, turn the key to the "on" position. Probe the back side of each fuse. There are two places to check. If the test light doesn't illuminate on either side of the fuse, that fuse is most likely blown. If the test light only illuminates on one side, you'll need to look in the owner's manual to see what that circuit feeds. Certain circuits require that you activate something to illuminate the test light-for example, turning on the headlamp switch to energize that particular fuse.

If your cruise control won't work at all, the next step is to make sure the servo operates freely. While squeezing the diaphragm with your hand, watch the throttle open and close. The cable should move freely, with no binding or tight spots.

There are two hoses going into the cruise servo. The smaller hose is a vacuum supply, and the larger hose is the dump hose. First, check the vacuum-supply hose using a vacuum gauge. It should read between 15 and 20 inches of vacuum at idle. If not, check the hose for leaks. This hose originates at the right side of the intake manifold under the fuel-rail cover. There you will find a solid black or a black-and-white vacuum check valve with three ports just below the fuel-pressure test port.

This is a one-way check valve that prevents a loss of vacuum to the A/C controls and cruise servo when the manifold vacuum drops under acceleration. To test the valve, remove the valve from the hose and apply vacuum to the port closest to the intake. It should not hold the vacuum. Next, apply vacuum to one of the smaller ports and cover the other small port with your finger; it should hold vacuum. (Remember that the vacuum hoses and check valve can deteriorate and become brittle with heat and age. This is a very common problem.)

If the vacuum supply is good, you need to check the operation of the dump hose. Use a vacuum pump to apply vacuum to the dump hose to approximately 15 inches, then press on the brake pedal. The vacuum should be dumped, and the gauge should read 0 inches of vacuum. (If the vacuum is not dumped, you'll need to check for a pinched vacuum hose or a faulty or out-of-adjustment brake vacuum-control switch.)

If vacuum is getting to the servo and can be dumped, check to see if you're getting power to the servo. While holding your hand on the servo, have someone turn on the ignition and turn the cruise control switch to the "on" position. You should hear and feel the solenoids in the servo click. This lets you know the switch is working and that you have power to the servo.

If you didn't hear or feel a click in the servo, you'll need to check to see if you're getting voltage to the servo itself. Disconnect the connector at the servo and connect a multimeter between terminals B and D. It should reed approximately 8 volts.

If you aren't reading 8 volts between terminals B and D, have someone release the steering-tilt arm and, with the key and cruise control in the "on" positions, move the steering column up and down slowly. If you are reading an intermittent voltage at the multimeter, the wiring from the cruise-control engagement switch could be broken inside the column. This problem can result when the wires become brittle with age. As you use the tilt-wheel feature, the wires bend and eventually break.

If you think you have a solenoid problem inside the cruise-control servo, there's a "no tools required" test you can perform. Simply pull the servo vacuum pod and cover the two vacuum ports with one finger, then push another finger against the dump servo plunger. The diaphragm should hold as long as your finger is on the plunger. If it doesn't, there's an internal leak in the servo or diaphragm, and the servo will need to be replaced.

There's a plunger and a pin inside the dump servo. Make sure they move freely and aren't stuck.

If you have a multimeter, you can use it to check the resistance of the solenoids inside the servo to make sure they're operating properly. The following procedure will work on most C4 servos. The connector will be labeled A, B, C, D, E. Unplug the connector; remember, you'll need to refer to the correct alphabetical locations at the servo.
1. Check the resistance between pins A and C. It should be somewhere between 30 and 50 ohms.
2. Check the resistance between pins C and E. It should be somewhere between 30 and 50 ohms.
3. Check the resistance between pins B and D. It should be somewhere between 10 and 30 ohms.
If any of the resistance readings falls out of range, a defective solenoid is likely to blame and the unit should be replaced.

If all of the above test out OK, check to see if the brake switch is out of adjustment. A quick way to diagnose this is to hold the brake pedal up with the top of your left foot while pressing the cruise-control button on a testdrive. If the cruise control tries to engage, the brake switch will most likely need to be adjusted. This is very common. Manual-trans Corvettes have a clutch switch that has the ability to disengage the cruise. This switch can be tested in the same way as the brake switch.

To adjust the brake-pedal switch, simply depress the brake pedal fully. Insert the brake-pedal switch until it seats on its bracket; you will hear it click as you insert it. After inserting the switch fully, pull the brake pedal back against its stop; you'll hear the switch click into its correct position. Reconnect the brake switch's electrical connector and vacuum hose (if equipped). Have someone check your brake lamps for correct operation before you go on a testdrive.

If you suspect that the brake switch may be faulty, you can use a test light to probe both sides of the switch. The test light should illuminate on one or both sides of the two wires, depending on the year of your car. If the light only illuminates on one side or the other, or both sides light up, the switch is OK. Next, press the brake pedal down and perform the same test. You should have the opposite reaction. If one side lit up and the other didn't on the first test, both sides should illuminate with the pedal applied. If both sides illuminated on the first test, only one side should illuminate when the pedal is applied. (If there was no change at all, then the brake switch should be replaced. If you have no illumination at all when you perform the first test, check for a blown fuse or an open circuit between the fuse box and the brake-light switch.)

If your cruise control and speedometer aren't working, the Vehicle Speed Sensor (VSS) could be the problem.

The VSS, which is mounted on the output shaft of the transmission, sends electrical pulses to the computer. A magnet spinning past a sensor coil generates these pulses. As you increase the speed of the vehicle, the frequency of the pulses increases. Usually, if this sensor has failed, it will leave a fault code 24 in the Engine Control Module (ECM). These codes can be retrieved using a scanner.

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